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Monday, April 16, 2007

Wireless Power

I have a number of friends who are seriously worried about my sanity. Not because I sometimes have the greatest ideas for anti-gravitating devices or clean nuclear power. Nah, they are used to that. In this case they worry because I spent last week shouting at my laptop. More specifically, I was cursing the whole internet! I was looking for some insights regarding the 'wireless power' that I read about. I actually found a lot of articles. But they are all more or less identical copies, and they all explain nothing - but that with an impressive amount of words.

After a week or so, I gave up. It then occurred to me maybe one or the other reader of our blog understands more about the technical details, so here is what I found out with a lot of open questions.

The issue is how wireless power for home use could work. That is, we are typically talking about distances of some meters or so, over which we want to transmit energy to power technological devices (say, your cellphone), preferably without roasting every human in the room.

To set the base, energy can of course be transmitted without wires. There are generally two ways how this can work:

One is to use electromagnetic radiation. E.g. your microwave does that. Actually, every bulb does that. The problem with this energy transport is if you want to use it over useful distances you either have a sender that broadcasts the energy into all directions, and a receiver that picks up only a part of it. This means a lot of energy is just lost. Or you focus the sender's radiation on the receiver, which means for moving targets you have to track them. (The microwave is a box that avoids radiation loss by reflection, and you don't want to sit in it).

The other possibility is using inductive coupling. In this case, energy is transferred from a sender to the receiver by using the very near field, and the radiation loss is negligible. This technique works pretty good and very efficiently and is in fact used to recharge many devices. The problem is that it only works on really short distances (say, a centimeter or so).

Neither of both ways sounds very useful for wireless power at home. Now, while I was scanning through these articles, I found out that there are currently two ideas on the market that rely on different schemes.

B) The other one is the technology used by a company called Powercast. They have a website that you find here. It it almost void of any information. They have a form that you can fill out and they send you some pdf-files. These again hardly contain any information (except some general explanation about the allowed limits on power density). If you don't want to fill out the form [1], the pdf's are here: 1,2,3,4. They apparently have presented their device on the consumer electronics show 2007, see e.g. here, here, here, or here (they are all more or less identical).

Let me first comment on A: The paper says they propose an 'efficient wireless non-radiative mid-range energy transfer'. Efficient means there is little energy loss. Mid-range means it potentially operates on the distances that we are interested in. Non-radiative means it doesn't use radiation. The idea that they build up upon is a resonance effect. You know that from your car. Your engine causes vibrations, and if you accelerate their frequency changes. If the frequency of the vibration coincides with the resonance frequency of some parts in your car (say the CDs in the glove box) they will also start to vibrate. This does extract energy from the engine, just that for the car this is a totally negligible (though annoying) effect.

Now the technique of A proposes to use a receiver and sender system that are resonant objects. In addition to that they state that they do not use the radiation field for this. Note that a field can very well be time-dependent (oscillating) without actually having an energy flow to far distances. For the sender and receiver they consider two examples: disks and loops. The paper does hardly contain any calculation, it seems there isn't very much one can do analytically with these boundary conditions. But electrodynamics can be treated numerically without too much complications, so that's what they have done. Figure 2 from physics/0611063 shows how that field would look like for the two disks (sorry, I removed the figure due to copyright reasons, see update note below).

Interesting is also Figure 6 which shows how the field gets distorted by a wall (to the right), and by a by sample (the square) that simulates a human (again, sorry, I removed the displayed figure, please check the paper) . In both cases, there is not too much distortion which is really promising. As they write 'the system performance deteriorates [...] only by acceptably small amounts'.

I was wondering why the 'human' isn't placed between the disks, wouldn't you too? But I have to say I find it actually reasonable. As long as you don't hit the resonance frequency you probably don't distort the field too much. A human body is very unlikely to contain resonating parts. Though I wouldn't want to have a pacemaker in such a room. More generally, one should ask how other technological (and metal) objects affect the field. Also, what does the field look like if sender and receiver are not parallel to each other?

But okay, I thought, great. So far I can make sense out of this. But this is the static configuration, in which case there is not really a sender or receiver. It's just two coupled systems. But to describe the realistic situation, one disk (loop) has to be the source that 'powers' the other one. The above figure is perfectly symmetric, so it can't describe this situation.

In particular the question is where is the energy flow localized in that case? I mean, energy is a locally conserved quantity. It has to get from the sender to the receiver somehow. My naive guess would have been, it takes place in that cylindrical part of space whose end-caps are the sender and receiver - this intuition relying on the simple fact that photons like to travel in straight lines. I was looking for something like the Poynting vector of that field, but couldn't find anything. The only 'explanation' I found was from Howstuffworks, which says:

"Electricity, traveling along an electromagnetic wave, can tunnel from one coil to the other as long as they both have the same resonant frequency. The effect is similar to the way one vibrating trumpet can cause another to vibrate."

This, excuse me, is simply bullshit [2]. We're talking about a classical system, energy doesn't just 'tunnel' somewhere. The vibrating trumpet transfers its energy via air molecules. Try it without air, you'll see. (You also find the tunnel-explanation in the BBC News article). In this article at physorg, you will find the statement 'Most of the energy not picked up by a receiver would be reabsorbed by the emitter' which doesn't make sense to me either. Photons don't just turn around and fly back if they were not absorbed.

The reason why this puzzles me is that their paper considers as an example a 'useful extracted power' (p. 15) of 10 W (p.16). Now I would expect this power to be transferred between the two loops, that is, it for a diameter of 30 cm, it is distributed over a surface of roughly 1000 cm2. Distributing the power over a surface that large does of course significantly lower the power density relative to that of a cable. But still one finds a power density of 10 mW/cm2(The FCC limit e.g. in the frequency range 30-300 MHz is 0.2 mW/cm2). One can of course make the loops larger, just that - if you ask me - already a diameter of 30cm doesn't appear so very handy to me.

In addition to this, I have tried to recall how these resonance effects work. In the symmetric configuration (none of both is a source), there is a phase shift between the oscillations of both coupled systems, and energy is transferred periodically from the one to the other. On the average, this does not lead to an energy flow. In case energy is 'used' on one side, an average flow will take place. However, the energy of the total field is typically significantly larger than the fraction that is transferred. I am not sure I understand all the details, but it seems to me that indeed the extracted energy is only a small fraction of the total field. Then, the power density of the total field is even larger than the above estimate, even though a large part of it does not lead to an effective energy flow, but just goes back and forth [3].

I looked at the slides from the talk, and it seems to me that the configurations examined there indeed have a source and a receiver. But since I didn't hear the talk I am not sure, and again I couldn't find anything about the energy transfer. So I wrote an email to the guy who posted the paper on the arxiv, Aristeidis Karalis. He kindly explained:"Think of two penduli connected with a spring. If you move one, energy will be moved to the other and then back and so on. The energy stays in the system and does not leak out. It just jumps from one to the other back and forth." I am not sure I can make more sense out of 'jump' than out of 'tunnel'.

I repeated my question on where the energy flow takes place, but it seems I exhausted his patience at some point (well, I know, I can be really annoying). Interesting is also what he wrote regarding my question why the human sample wasn't placed between the plates:

"The system of dielectric disks is more affected from extraneous objects than the system of loops. I initially made calculations for the 'human' between the two disks, and the numbers were still viable but worse. Therefore, I chose the positioning presented in the paper, because for application where humans are present most probably the loops would be used, while for applications where disks would be used (e.g. optical regime) the materials have much smaller indices and losses."

But as I said above, I actually believe that a human wouldn't make a big distortion because it's unlikely to hit the resonance frequency.

I then looked who's who on this photo, and I thought maybe it would be more helpful to ask Dr. Marin Soljacic. So I wrote him an email, but he didn't reply - at least not yet[4]. And this is where this story ends.

Then let me summarize what I think about A. If there really is very little energy loss, then it seems to me this energy flow has to take place around the axis between sender and receiver and is roughly distributed over a surface of their diameter. If the efficiency of that is indeed almost independent of the sender's and receiver's relative positions and orientations, this means it is somewhat like an automatically working tracking mechanism. The problem is then that the energy density shouldn't be too high between sender and receiver. (Or you'd want to make sure you don't get in the way.) I am not too good with numbers (famous for loosing factors of 106 or so). So I don't know - given the limits on the power density are fulfilled - how long would it take to charge the average device [5]?

Now to B: In essence the idea is using a broadcaster that operates in 900-MHz range with acceptably small power density. They call that the 'omnidirectional power beacon' and it 'will recharge devices within about a 1-meter range' [source]. This energy can be received by a device they call 'power-harvester'. Since there are constraints on the allowed power density, the field can not be too large which means one can only use it to power really small devices. As they say:

"We have a technology that's here today, with FCC approval, that sends RF signals through the air to power very low- power devices directly or to recharge battery-powered devices," said Powercast vice president Keith Kressin. "Our wireless systems can recharge batteries in any consumer device smaller than a cell phone, from up to a meter away." [source]

It seems they actually have working products, and had a demonstration that impressed many people last month. I have to say though, I don't think I would want to work in an office where LED lights start gleaming through energy they extract from the radiation field around me. No matter if you tell me the limits are compatible with what the government demands.

"You can forget their orientation, forget the use of coils; just watch the LED get brighter the closer you place your device to the Powercaster."

This technology seems to have been developed mainly by Dr. Marlin Mickle and collaborators from Pittsburgh. I checked some of his publications, to find out how efficient this power transfer would be. I found a lot technological details about antennas, but not what I was looking for (I could not access all of the papers). If you do better than I, please let me know.

Besides me feeling uneasy with sitting in that power transmitting field, my problem with B is that I am afraid there might be a considerable loss into radiation. In particular, unlike this article from the Alternative Consumersays, this can hardly be very 'green' . The Alternative Consumer essentially repeats Powercast's information sheet that says nice things like 'Powercast Reduces High-tech Waste' because 'Continuous recharging of batteries via the Powercast Wireless Power Platform has the potential to reduce the huge waste stream of batteries to a mere trickle'. Indeed, instead of rechargeable batteries you then use the power-beacon and -harvester, and instead of transmitting power with negligible loss via a cable you radiate it generously into your apartment where most of it goes byebye to outer space.

In addition, I wonder what happens if the guy in the apartment below me installs a 'power beacon' it his ceiling. And my neighbor to the right. And to the left...

To summarize: I wouldn't buy neither A nor B.

Update,April 17th: Yesterday, I sent an email to one of the authors of physics/0611063, Aristeidis Karalis, asking whether it is okay that I display the figures from paper. He replied that the paper is in the publication process and asked me to remove the figures. I was kind of afraid that would happen. So, I am sorry for the inconvenience, but you'll have to look at the pdf-file.

Footnote 1: You can fill in the address fields with x, it works. They sent only the requested files and no spam.

Footnote 2: The use of the expression 'tunneling' is most likely due to a misunderstanding. The electromagnetic field configuration of the proposed system makes use of the Whispering Gallery modes which have an exponentially decaying tail. From the solution of the wave-equation this is similar to the tunnel-effect in quantum mechanics. Just that in electrodynamics the amplitude is that of the electromagnetic field and not - as in quantum mechanics - a probability amplitude. The typical 'tunnel effect' in which a particle 'jumps' through a classically forbidden region has nothing to do with the above described resonance.

Footnote 3: As Stefan pointed out in this comment, the ratio between the transferred power and that of the total field is of order 1000, which means the total power density would be in the kiloWatt range - far above the allowed FCC limits.

Footnote 4: Yes, I checked the junk folder. I found it indeed possible that PI's highly efficient filter discards MIT-senders as spam.

Footnote 5: As my husband just taught me, with a power of 1.5 W it takes 2 hours to charge a common battery of type AA. That is, the transmitted power they considered is very realistic to applications, and going below it makes the scenario considerably less appealing. To meet the limits on the power density, you either have to wait 100 hours, or increase to diameter of the loops to a meter or so.TAGS:WIRELESS POWER, ENERGY, PHYSICS

I am sure that you know abt Nicola Tesla. He wanted to do just that: wireles transmission of energy.

He too became real god at roasting birds in mid-air..

(a real shame that Tesla and Einstein never got in touch, (as far as I know))

It is absolutely fantastic what Tesla foresaw already in oktober 1919:

Quote:

The interconnection of existing telegraph exchanges or offices all over the world;

The establishment of a secret and non-interferable government telegraph service;

The interconnection of all present telephone exchanges or offices around the Globe;

The universal distribution of general news by telegraph or telephone, in conjunction with the Press;

The establishment of such a "World System" of intelligence transmission for exclusive private use;

The interconnection and operation of all stock tickers of the world;

The establishment of a World system—of musical distribution, etc.;

The universal registration of time by cheap clocks indicating the hour with astronomical precision and requiring no attention whatever;

The world transmission of typed or handwritten characters, letters, checks, etc.;

The establishment of a universal marine service enabling the navigators of all ships to steer perfectly without compass, to determine the exact location, hour and speak; to prevent collisions and disasters, etc.;

The inauguration of a system of world printing on land and sea;

The world reproduction of photographic pictures and all kinds of drawings or records..."

Unquote, from:

http://www.lucidcafe.com/library/96jul/teslaautobio.html

I would have liked to go "wiki" on the subject, but as I am currently in China this does not seem to work! luckily Bee's Blog is still available :-))

*argh* the taxes... i forgot. is it the same day in canada? was I supposed to file in something? Would someone have told me? Should find out... what a way to start the day. gnagnagna... thanks...

Hi Klaus,

yeah, Tesla was the real thing :-) All that 'wireless power' is exactly what he has also done. I admittedly don't quite get all that fuzzing around. There is a very simple fact one can't avoid: if you want to get energy from point X to Y it has to go through every point between X and Y (in a flat space on a straight line). If you have a limit on the allowed energy density, there is nothing you can do about it. Full stop. It doesn't matter how fancy you make the setup, and if you call that Whispering Gallery Modes or swhatever.

An apparatus for a wireless power supply including a mechanism for receiving a range of RF radiation across a collection of frequencies. The apparatus includes a mechanism for converting the RF radiation across the collection of frequencies, preferably at a same time into DC. A method for a wireless power supply including the steps of receiving a range of RF radiation across a collection of frequencies. There is the step of converting the RF radiation across the collection of frequencies, preferably at a same time into DC.

yes, thanks. I should have mentioned the potential application for medical devices, I found that very promising indeed. It is just not clear to me if one can avoid that excess heat is emitted into the tissue around the 'harvester'. Right now while sitting here my laptop's battery is getting warmer and warmer. Since I couldn't find any details on whether they have measurements on the temperature of the harvester during charging, I thought I'd better not go into it. Best,

B.

PS: Overall seen I find the whole topic really poorly documented, even what I could find as 'scientific' publications.

Actually, I wonder if all the things we already have transmitting through the airwaves might be more responsible for climate changes than CO2, if you want to believe that humans are causing some climate changes, etc.

maybe Tesla was somewhat overexited with his own ideas on powertransmission and comunications. However he was a genius just as bright as Einstein. tesla was a true engineer with a "can do" attitude. Had he lived 60 yrs later Bill Gates would have been merely a programmer on his team.

I saw a discription of a space elevator povered by a laserbeam precicely aimed from the ground station. (on German ZDF actually)

Do you think it feasible to aim a beam carrying 100KW+ netto at a target of abt 4m2 over a distance of 400 KM? I doubt that!

For the magnetic induction principle: Maybe a good idea for electric battery driven cars! just imagine touch-less batterycharging when parking,or waiting for green light. This is not out of this world.

btw: China is great in many sence of the word. In the soviet they build a perfect communist society for 10.000 people (being rich and driving German cars) out of a 150M population.

In China they build a perfect communist society (being rich and driving German cars) for 300M people out of 1,2B.

For the magnetic induction principle: Maybe a good idea for electric battery driven cars! just imagine touch-less batterycharging when parking,or waiting for green light. This is not out of this world.--------

I was going to mention this, that electric cars with AC induction motors are using "wireless power". A friend of mine from grad school (Martin Eberhard/MS Elec Eng, UIUC) got rich & started Tesla Motors. Elon Musk (of Paypal fame, was a Stanford physics grad student) is an investor.

A recent entry on Tesla Motors blog mentions Feynman, written by a Caltech alumni: Tesla Motor's "Principal Power Electronics Engineer". They are using the AC Propulsion's power module (founded by a famous Caltech alumni A. Cocconi..whose parents were CERN particle physicists). I run into A. Cocconi & Feynman's son-in-law at the Rose Bowl, they both fly R/C electric planes.

The 900 lb battery pack, which degrades over time is a big issue with EV users. Given that Tesla is loaded with EE & physicists (Caltech, UIUC, Stanford, et al) they probably would be receptive to "wireless power". Martin is a fan of Tesla.

I am really relieved to see you haven't just dropped onto the keyboard or something ;-)

Regarding global warming etc, I doubt that the electromagnetic radiation is responsible for that. There can't be very much absorbed by air molecules - if it were the case, it wouldn't work over long distances as it does.

I find it always weird to think about how many phonecalls and emails might just fly through my head. It's really spooky. I really wouldn't want to have such power-beacon in my appartment. Best,

I could see short range power transmission working; using a similar engineering to cell phones, multi-antennae spread spectrum broadcasting system. The broadcaster could target spread spectrum beams at multiple receivers, each with their own set of resonance patterns.

If you spread the energy across a broad enough set of frequencies your actual power transmitter could broadcast below the black body radiation of the environment. This doesn't mean that the receiver could extract power from the black body radiation however. Because the black body radiation comes in a mix of phases the net effect would be to impart no additional work to the receiver. Only a broadcaster that can hit the phases of the resonances correctly would be able to impart work to the receiver.

Really it best to think about this in terms of information theory. What do the braodcaster and receiver have to agree upon to do work? In this case phases, frequencies, and directions.

the technology is working towards:"But researchers believe that once silicon circuits slim down to 10 nanometers, which the semiconductor industry predicts will occur after 2020, they will start leaking electricity profusely."

The words "start leaking electricity profusely" sort of jumps out at you!

I will delve into this subject bee, it seems a near feasible concept, best pv.

that's a very interesting and infromative post :-), and thanks for the link to the Soljacic/Karalis/Joannopoulos paper! The paper could have a little bit more details about the actual field configurations, for my taste, and perhaps an equation or two describing the field... But perhaps this is all evident to electrical engineers or people doing microwave and optical resonators...

But the physics involved in this concept is really great basic stuff: funny solutions of Maxwell equations, these whispering gallery modes, coupled resonators, overlap integrals for the coupling, linear perturbation theory...

Your point about energy densities involved is indeed troubling. Maybe the construction is supposed to work at a much lower frequency, where the allowed power densities are higher? I didn't find any hint about the actual frequency range in the paper.

And then, there is something that worries me even more: These coupled resonators are supposed to have very high Q-factors, on the order of 1000 and larger, if this scheme is supposed to work best. But this means that the energy which is constantly being exchanged between the coupled resonators is 1000 times larger than the energy that is "used" at the detecting device. If the netto transmission is supposed to be about 10 Watt, the power acutally constantly exchanged is 10 Kilowatt! That's uncredibly high!

Is there a very stupid mistake I am making?

Or is there indeed some "tunneling" of energy involved, which circumvents the continuity eqution? I can't believe that... I am a little bit puzzled, and must say I have no good intuitive idea about the "non-oscillating evanescent field". The acutal distribution of energy flow/power density through a plane normal to the connecting line between sender and receiver at different phases of the resonance oscillation would be quite interesting...

depends on which technology you talk about. Regarding the Powercast stuff, it might very well be that one can downscale their products with new technologies. (Though I am not completely sure how small you can shrink something that essentially has to act as an antenna.) Regarding the MIT proposal, I don't think it helps to shrink it. For one, they have examined the transfer only over distances of a few (say up to ten) times the diameter of the disks (loops). Besides this, if you shrink the extension of these things, but still want to transfer the same amount of power then the power density goes up, and I am afraid one would run into conflict with the FCC guidelines. Best,

In Germany, this is a big controversial topic since quite a long time - search for Elektrosmog, or Electromagnetic radiation hazard. There are many people who are convinced that they have become ill because they live nearby power lines, or cell phone antennas.

This study on the influence of electrosmog on bees ;-) by Jochen Kuhn and Hermann Stever seems a little bit to be in this "German tradition", and I have to admit I have mixed feelings about its scientific merits. Anyway, there seems to be no peer-reviewed publication yet, and that a mysterious malady that is causing honeybees to disappear en masse from their hives in parts of North America and Europe may be linked to radiation from cellphones seems to be quite specultative to me. I am not alone with this judgement ;-), there is a quite sceptical article about this in Spiegel online (unfortunately, only in German).

Anyway, the upper limits for electromagnetic energy densities are just set to avoid potential hazards, which are not completely known yet.

I think you have hit the crux of the problem. The resonators of the sender and the receiver already have to be oscillating a head of time with a really high engery density to reliable transfer a small amount of energy. Now it is not to hard to shield the surrounding environment from the resonators, and then pass an antenea through the farday cage. The big problem is getting the oscillators moving in the first place. So, its not a matter of the resonators exchanging large amounts of energy, but rather having stored ahead of time a large amount energy, to reliable detect the modest transfer of energy.

I have to say though I find it somewhat implausible to connect this to electrosmog. This radiation has been around for several years, but the bee-problem happened really suddenly and really fast. If it was caused by electrosmog, I'd have expected it should have happened more gradually over the last years. Such a sudden collapse indicates to me it might be a problem in the supply chain that only now might have reached the bees or something (I doubt its directly correlated to climate changes though I guess that's what people will eventually settle on).

These coupled resonators are supposed to have very high Q-factors, on the order of 1000 and larger, if this scheme is supposed to work best. But this means that the energy which is constantly being exchanged between the coupled resonators is 1000 times larger than the energy that is "used" at the detecting device. If the netto transmission is supposed to be about 10 Watt, the power actually constantly exchanged is 10 Kilowatt! That's incredibly high!

Thanks Stefan for digging out the relevant factor! That's what I meant to say with the somewhat vague paragraph

However, the energy of the total field is typically significantly larger than the fraction that is transferred. I am not sure I understand the details of the paper, but it seems to me that indeed the extracted energy is only a small fraction of the total field. Then, the power density of the total field is even larger than the above estimate, even though a large part of it does not lead to an effective energy flow, but just goes back and forth.

Bee, considering there are still uncomfirmed 'health risks' from mobilr phone masts, and from living near electricity pylons

I think significant electricity transfer, over significant distances in homes - has many limits and risks - especially to humans walking 'in between'

Never mind worrying about kids putting their fingers in a 220V socket, how about putting kids in (inside) a microwave.

b-yen charging electric cars when parking or at traffic lights, or charging electric trains on rails, is 'not out of this world.'

The great thing about Radio controlled planes whether petrol or 'electric' is that you can radio control them - but the petrol or electric charge will die sooner or later - so best learn how to land safely to 'refuel' - unless of you are using solar power to recharge any batteries

A brief note: I just got an email from that guy with the paper, and he asked me to remove the figures. Since the paper is on the arxiv, it's just an annoyance cause you'll have to open the pdf-file now.

There is something fishy about the authors numerical solutions. Their side plot is differentiable across the disk/air boundary, this does not seem possible because the dielectric permittivity is not differentiable on the boundry, it is a step function.

The energy denisty or field inside the disk should be much higher, to match the high quoted Q-factors.

I was just thinking of "wireless internet. That if you didn't have a "good security system" then what said people couldn't tap into your signal and piggy back your internet?

Also was thinking about tachyon/graviton condensation and other thoughts about "heterodyne solutions."

heterodyne adj : of or relating to the beat produced by heterodyning two oscillations v : combine (a radio frequency wave) with a locally generated wave of a different frequency so as to produce a new frequency equal to the sum or the difference between the two

Gr from string theory.

The idea of crossing any "field" and then having something "precipitate" from it seems interesting.

The power line issues were somewhat of a interest to me as well and it's effect on biological systems.

Use a florescent tube to walk under a high energy line, or how about the effect of something levitating within the strong field created.

The Bee story is troubling.

:)Internet can also piggy back hydro lines, beside using the ole dial up or cable.

The problem of energy transport, as far as I can see, is similar to the fact that our Sun gives out a vast pool of Photons, and gives its energy to the Earth. The fact that the intervening space between the Earth and Sun is not Heated up in any way (remains at a constant cold temp)..obvious because there is nothing but space vacuum, and maybe the odd space module.

So how about some "phase" impacting lasers?

http://en.wikipedia.org/wiki/Photomultiplier

Works for photons, so how about an Electron Multiplier ?..which fits inside the devise's, and is triggered by a specific light wavelength?

Like the remote control of your DVD player, if this strikes the scintillator, then instead of a photocathode, maybe a "phase" disciminator could be developed along the lines of Graphine Conductor?

Its not about sending Electricity through the air, its about creating devises that are electrically self generating, or to almost that effect.

This is a very nice post. I enjoyed reading comments as well. I was going to mention Tesla but others mentioned it already. I believe that in this case, legacy notions such as Maxwell's equations are hindering progress.

Maxwell, by the way, believed in ether and his equations were meant to prove the existence of ether.

In this case a medium such as the ether seems to make sense. Although it needs a new label to make it acceptable to physicists as a fine grained medium, not an absolute frame of reference.

There is something fishy about the authors numerical solutions. Their side plot is differentiable across the disk/air boundary, this does not seem possible because the dielectric permittivity is not differentiable on the boundry, it is a step function.

You mean Fig 1, right side? Not sure here, doesn't that depict only the absolute value of the field component that 'points out' of the plane (in the left figure)? Best,

This has been bugging me a lot as of lately as well, and been studying Tesla more because of it. it seems when in a 900mhz electromagnetic field, you have access to 1 watt of energy... and the 900mhz band goes though walls and such pretty easily. I am trying to find a way to receive the 900mhz wavelength, convert it to energy and light an led. if anyone has any ideas on how to do this, id be a happy boi. there must be a way to do it.

I have posted the following yesterday to sci.electronics.design Maybe it's of interest for some readers here too.

Re: WTF ! MIT to eliminate power cords ?

Hello,I tried to calculate some numbers and here are my results.

As supposed by the "inventors", the coils are used in resonant modeat medium wave frequency (1MHz or higher). Only the B-fieldis used to transfer power.

A reasonable effeciency will require a high Q of let's say 1000. Thisalso means you have to tune the resonance frequeny of your receivercoils with high precision, e.g. 0.2e-3.

The sender coil has an inductance of some ten mirco Henries.By the way the coupling factor is in the range of 1e-4 to 1e-3 in theproposed distance with a receiver coil of the same dimensions as the sender coil.

The voltage along the coil maybe Kilovolt(s) due to the resonance.This means strong E-fields close to the coil too.

It's true that the unwanted transferred power into non-tuned loops canbe neglected regarding power in the proposed distance. This may notbe true very close to the coil.

The B-field is below the earth magnetic field but it's alternating with MHz.So it doesn't make sense to compare it this way regarding the possible effctsto humans and devices.

Who wants very strong E-fields and B-fields close to the coil in a living room?